Bone-Like Mineral and Organically Modified Bone-Like Mineral Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 6

surface wettability by water. Wettability is the relative adhesion of a fluid to a solid surface.
In the case of biomaterials and immiscible fluids, wettability refers to the ability of water
to spread or adhere on an implant surface. A wettable surface has surface free energy 10
dyn/cm greater than the surface tension of the liquid. Hydrophilic surfaces that exhibit
enhanced wettability can improve osteoconductivity by providing energetically favorable
binding sites for integrins (Kilpadi and Lemons 1994; Rupp et al. 2006). Hydrophilic sur-
faces that exhibit enhanced wettability can improve osteoconductivity by providing ener-
getically favorable binding sites for integrins (Kilpadi and Lemons 1994; Rupp et al. 2006).
An implant's surface charge resulting from dissociating ions can also have an effect on
biointegration. With more dissociating surface ionic groups, oppositely charged biomol-
ecules become electrostatically attracted to the surface. For instance, the spontaneously
formed TiO 2 film on titanium implants reacts with water to form acidic and basic hydroxyl
groups at the surface that enhance surface charge and protein adsorption (Kilpadi and
Lemons 1994). Increasing the hydrophilicity improves osteoconduction by increasing
osteoblastic cluster formation compared to unmodified titanium surfaces (Rupp et al.
2006).
Surface topography. Cells interact with the ECM through transmembrane focal adhesion
kinases that allows them to transduce external cues through the cytoskeleton into the
nucleus to induce transcription. Transduction of external mechanical cues elicits specific
biochemical signals controlling cell cycle, proliferation, migration, and differentiation.
Surface topography and elasticity are key factors that can control and direct this cellular
response (Ingber 2006, 1997).
Implant surface topography has been extensively studied to identify correlations
between surface structures and fixation to bone. Despite the heterogeneity in experimental
methods, there is a positive relationship between surface roughness and bone to implant
contact (Shalabi et al. 2006). Several approaches to modify surface roughness at the micron
level have been utilized, among which sandblasting, acid etching, and sodium hydrox-
ide treatments are the most widely used (Bollen, Lambrechts, and Quirynen 1997). For
instance, osteoblasts cultured on sandblasted implants exhibit enhanced mineralization
compared to osteoblasts grown on smooth surfaces (Marchisio et al. 2005). However, there
is an upper limit to roughness for improving tissue integration or inducing an enhanced
cellular response on the order of R a = 4 μm (Rønold, Lyngstadaas, and Ellingsen 2003).
Roughness is therefore an important design parameter to consider when altering surface
characteristics to enhance osteointegration.
Surface chemistry and surface topography are also co-optimized to enhance osseointe-
gration. For instance, sandblasted acid etched implants are contaminated by hydrocar-
bons minutes after exposure to air, making their surface chemistry hydrophobic. These
implants are processed with nitrogen gas and stored in NaCl solution to decrease con-
tamination and increase hydrophilicity. This surface modification procedure results in
improved osseointegration (Rupp et al. 2006).
Micro- and nanostructuring techniques are also used to control molecular-level interac-
tions between cells and the environment: soft-lithography, photolithography, sputtering,
self-assembling nanostructures, and physical and chemical vapor deposition modify sur-
face topography at the micro- and nanoscales and control cell behavior (Tan and Saltzman
2004; Martinez et al. 2009; Dalby et al. 2007, Dalby et al. 2004; Xia and Whitesides 1998).
Surface micro- and nanotopography regulate cell orientation, morphology, and cytoskel-
etal rearrangement and promote cell adhesion, proliferation, and differentiation (Martinez
et al. 2009). For instance, to identify the role of surface topography in directing osteogenic
differentiation, human mesenchymal stem cells were grown on 120-nm grooves created